How can you promote unity in a nation?

Funamentally, there has NEVER been "peace" the world over. Conflicts have a huge variety of causes and methods, most of it rooted in the fundamental nature of humans to compete over limited resources, and to dream up ways to divide humanity into groups of "us" vs "them".

The question is unanswerable, because we simply don't understand the reasons why conflict occurs in the first place well enough to say: if the conflict is caused by X, we do Y to stop it.

Even for maintaining large portions of the world at peace, there is no good formulae, because everything that's been tried eventually fails. This is not to say that we're doomed to always fail, but that expecting a formula for "peace" which can't adapt to changing times WILL always fail eventually.